1. Field of the Invention
The present invention relates to an ink jet device including an actuator that ejects ink droplets by changing an internal pressure of a pressure chamber.
2. Related Art
There has been known a piezoelectric type ink jet printer head including a cavity plate formed with a pressure chamber and a piezoelectric element positioned adjacent to the pressure chamber. In this type of printer head, an ink droplet is ejected from the pressure chamber when a driving pulse is applied to the piezoelectric element. The rising edge of the driving pulse displaces the piezoelectric element, thereby increasing the volume of the pressure chamber. This volume change decreases an internal pressure of the pressure chamber, and a resultant negative pressure is maintained for a predetermined time, which is equal to the pulse width of the driving pulse, so that ink is introduced into the pressure chamber from a manifold. Then, the lowering edge of the driving pulse releases the displacement of the piezoelectric element, whereby the increased volume of the pressure chamber is restored. This increases the internal pressure of the pressure chamber and ejects an ink droplet through a nozzle onto a recording sheet, which is being transported relative to the printer head.
The pulse width of the driving pulse determines the amount of pressure that contributes to ink ejection, which in turn determines the volume of the ink droplet. Because a color-scale of resultant images is determined by the volume of ejected ink droplet, it is possible to obtain a desired color-scale by controlling the pulse width.
However, when a driving pulse with a different pulse width is applied in order to change the volume of ink droplets, the ejection speed of these ink droplets also differs, because the pulse width determines a timing at which the pressure change inside the pressure chamber is superimposed on a pressure that restores the deformed condition of the piezoelectric element. When ink droplets are ejected based on predetermined timing-clock signals at different ejection speeds toward a recording sheet that is moving relative to the printer head, impact positions of these ink droplets on the recording sheet will be out of alignment, adversely affecting quality of resultant image.
It is an objective of the present invention to overcome the above problems, and also to provide an ink jet device capable of forming dots on desired positions even when the volume of ejected ink droplets varies.
In order to achieve the above and other objects, there is provided an ink ejection device including a cavity plate formed with a pressure chamber and a nozzle, an actuator that fluctuates an internal pressure of the pressure chamber to eject an ink droplet from the nozzle onto a recording medium, and a driving unit that selectively outputs a driving signal to the actuator. The actuator fluctuates the internal pressure in response to the driving pulse. The driving unit outputs the driving signal for ejecting a smaller ink droplet at a first timing earlier than a second timing for a larger ink droplet.